10 ms·
Celestial Navigation for Drones
- jcims 2y agoI didn't see an explanation of what strapdown meant in this context, so I dug one up: "Traditional, stable-platform navigation systems commonly involve separate accelerators and fibers or laser-based gyroscopes, with all the components mechanically and rigidly mounted on a stable platform that is isolated from the moving vehicle. This leads to the drawbacks of large size, poor reliability, and high cost. In contrast, in strapdown navigation systems, the inertial sensors are fastened directly to the vehicle’s body, which means the sensors rotate together with the vehicle. " https://www.mdpi.com/2504-446X/8/11/652 https://www.mdpi.com/2504-446X/8/11/652
- plasticchris 2y agoOr in short, the sensors are strapped down to the platform being measured - like your phone’s sensors for example.
- boscillator 2y agoYes! It's in contrast to gimbaled systems. Putting the measuring instrument on a gimbal simplifies the math and often improves accuracy, but at the expense that you need this large moving object that needs more power.
- imglorp 2y agoGyros on gimbals have other drawbacks, such as drifting and gimbal lock.
- anothertroll456 2y agoJust gimbal lock. Drifting happens to all of them. But also the gimbal mechanisms, gimbal low response time, etc.
- UltraSane 2y agoThe ultimate example of this is the incredibly accurate and expensive and complicated floating Advanced Inertial Reference Sphere used on the Peacekeeper ICBM. https://en.wikipedia.org/wiki/Advanced_Inertial_Reference_Sphere https://en.wikipedia.org/wiki/Advanced_Inertial_Reference_Sp...
- sandworm101 2y agoExcept that a gimbaled system, if enclosed in a single box/pod, can also be described as a strapdown system. The term speaks more to separate modularity than how the system functions internally.
- kragen 2y agoI wonder if you could dangle the star tracker below the drone on a long string, decoupling its attitude from the attitude of the drone. A kevlar or spectra string capable of supporting 100 grams would be 20μm in diameter; 3 meters of it would weigh a milligram, which is significantly less than gimbals. A small weight a couple of meters below the star tracker on a carbon-fiber-composite rod would seem to be able to stabilize its attitude further except in yaw.
- adolph 2y agoI wonder if GPS and the like will be used more for their clock features than for position. The emissions celestial bodies are perfect fiducial markers [0,1], but connecting them to position still requires accurate timekeeping [2], as the paper notes: Provided the use of an accurate clock, the results presented in this paper will not degrade over time. 0. https://www.twz.com/17207/sr-71s-r2-d2-could-be-the-key-to-winning-future-fights-in-gps-denied-environments https://www.twz.com/17207/sr-71s-r2-d2-could-be-the-key-to-w... 1. https://timeandnavigation.si.edu/multimedia-asset/nortronics-nas-14v2-astroinertial-navigation-system https://timeandnavigation.si.edu/multimedia-asset/nortronics... 2. https://www.rmg.co.uk/stories/topics/harrisons-clocks-longitude-problem https://www.rmg.co.uk/stories/topics/harrisons-clocks-longit...
- KineticLensman 2y agoThey are perfect markers only as long as you can see them. Clouds and fog are your enemies here
- mpenet 2y agoThat. However that works just fine for ICBMs and the like… The future is more likely to be quantum accelerometers and quantum gyroscopes, as they have no “external dependency”.
- cyberax 2y agoMore likely image-based navigation, where you just upload the entire imagery of the route and then use it to correct your inertial references. Encrypted positioning information from low-orbit satellites is another option.
- mpenet 2y agoThat doesn't work well in some conditions, it's not new either. Some cruise missiles that have TFR (Terrain-following radar) and actually do this already. It also is not really applicable when you are on a balistic course at *very* high altitude, course correction has to happen early in these case given the reentry speed/constraints.
- areoform 2y agoPerhaps I am too paranoid, but I've been told to avoid doing any DIY in this field of study. Apparently, or so I'm told, out of the many, many ways to end up on a list — building a working celestial navigation system can lead to some very inconvenient outcomes. Second, only to ordering large quantities of certain chemicals online. Is this true? ——— EDIT - from the paper, this is incorrect, > The introduction of GPS caused the interest in celestial navigation to wither due to its relative inaccuracy. Consequently, celestial navigation is primarily seen only in space-based systems, whose orientation must be known to high levels of precision. Nonetheless, celestial navigation was identified as a desirable alternative to GPS [2], primarily due its robustness against potential jamming. Critically, few GPS-denied alternatives exist that are capable of using passive sensors to estimate global position at night or over the ocean. For this reason, celestial navigation remains an important topic of research. The US and other militaries never stopped using these systems. They just stopped talking about them as much. Here's a literature search showing some of the slow & steady research on the topic, https://scholar.google.com/scholar?q=astro-inertial+navigation&hl=en&as_sdt=7,39&as_ylo=2000&as_yhi=2015 https://scholar.google.com/scholar?q=astro-inertial+navigati... Example systems that have been deployed in many (most? all???) American combat aircraft, https://theaviationist.com/2021/09/10/lets-have-another-look-at-the-b-2s-air-data-ports-and-astroinertial-navigation-system/ https://theaviationist.com/2021/09/10/lets-have-another-look... https://www.gpsworld.com/honeywell-demonstrates-military-grade-alternative-navigation-tech/ https://www.gpsworld.com/honeywell-demonstrates-military-gra... https://ieeexplore.ieee.org/document/290940 https://ieeexplore.ieee.org/document/290940 Alright. I'm ready to be on that list, Mr NSA agent.
- wolfram74 2y agoI've also been told learning too much about linux or the nuclear reactions in power plants or bombs puts you on a list. I just assume I'm on several.
- munchler 2y agoLearning too much about Linux puts you on a list? That can't be a thing. Isn't Linux itself entirely a civilian project?
- maxglute 2y agoCtrl+F and 0 results for munitions or bombs. Seems like this is really about $25 controller gets drones to within 4km in GPS denied enviroments, after which a $50 infrared camera + DSMAC find targets to hit.
- the__alchemist 2y agoI would assume the same. Operation in GNSS-denied environments is critical for military navigation systems. Comparatively, for civilian uses, it's an addon that provides low accuracy, and potentially high development or equipment cost (Maybe not for a cel nav camera, but for Ring Laser Gyro INSs etc) GNSS is very accurate, and receivers are cheap, but its reliant on satellite signals makes relying on it a liability in adversarial uses. Cel nav isn't self-contained in the way an INS is, because you need a clear LOS to the stars. But, it's useful on a clear night when your GPS is jammed.
- mapt 2y agoThanks for the summary. I suspect you could get this to FAR higher accuracy if you combined it with a recent upload of Starlink et al LEO constellation ephemera, an initial GPS fix at launch, and a planned flight path, because LEO constellations are bright foreground objects (high location-specific parallax differences against background stars) at apparent magnitude of about 5.0. This is simultaneously not reliant on perfect vertical attitude sensing coming off the autopilot IMU, you can do it purely photometrically. The limitation is that this is a dawn/dusk thing, in the middle of the night there isn't a ton of light reflected and in the day you're limited by scattered daylight. EDIT: Medium orbit satellites outside Earth's umbra but within view still provide some sort of visual fix. I wonder what the math is like for the GSO belt at midnight? EDIT2: Or the Moon.
- jcims 2y agoThat's a great idea. In the earlier days when they had about 2500 satellites in LEO I built a small visualizer from the fleet TLE data and it was remarkably simple with the skyfield library. If you're in the fringes of a GNSS denial area ADSB might be useful as well. Would need more hardware of course.
- alexpotato 2y agoFun fact: The SR-71 and U2 planes had automated celestial navigation systems b/c GPS wasn't around when they came out. There a story in the book about Lockheed Martin's Skunk Works where they mention turning on the system while one of the planes was in the hangar and it locked on to a hole in the roof (sun was shining through the hole and system thought it was a start).
- perihelions 2y agoAnd, it's a bit older than that: the SR-71's derived from ICBM targeting systems, https://en.wikipedia.org/wiki/Missile_guidance#Astro-inertial_guidance https://en.wikipedia.org/wiki/Missile_guidance#Astro-inertia... ("the latter of which was adapted for the SR-71...") (Actually the very first one, in that history, was an intercontinental cruise missile—a jet weapon that slightly predated (~1958) rockets powerful enough to cross oceans. ICBM's came a bit later. I'm pretty sure the first generation were pure-analog circuits, but I forgot where I read about that).
- alexpotato 2y agoReminds me of the "the distance between the rails of a railway are due to the width of Roman horse drawn carts" story.
- littlestymaar 2y agoIsn't that one a hoax though?
- all2 2y agoThere are a few standards for rail-line widths. I know the US is on one standard (I think the narrow width lines died out almost 100 years ago at this point). I know that Europe has two, or maybe more. https://en.wikipedia.org/wiki/Standard-gauge_railway https://en.wikipedia.org/wiki/Standard-gauge_railway << This makes for fun reading if you're interested in that sort of thing. Relevant passage A popular legend that has circulated since at least 1937[8] traces the origin of the 1,435 mm (4 ft 8+1⁄2 in) gauge even further back than the coalfields of northern England, pointing to the evidence of rutted roads marked by chariot wheels dating from the Roman Empire.[a][9] Snopes categorised this legend as "false", but commented that it "is perhaps more fairly labeled as 'Partly true, but for trivial and unremarkable reasons.'"[10] The historical tendency to place the wheels of horse-drawn vehicles around 5 ft (1,524 mm) apart probably derives from the width needed to fit a carthorse in between the shafts.[10] Research, however, has been undertaken to support the hypothesis that "the origin of the standard gauge of the railway might result from an interval of wheel ruts of prehistoric ancient carriages".[11]
- UltraSane 2y agoI read that the US military wants a modernized version of celestial navigation to reduce dependence on GPS. With modern light amplification technology it might be able to work during the day.
- pclmulqdq 2y agoThey have some of these on ships already.
- Havoc 2y ago400 Bucks Sensors is a touch rough. This would only work at night, right?
- littlestymaar 2y agoThe full title is: > An Algorithm for Affordable Vision-Based GNSS-Denied Strapdown Celestial Navigation Emphasis mine. In what kind of context do you expect drones to operate in an area where GNSS is disabled by electronic warfare devices? Do you really think that a $400 cost is of any issue for military use?
- Havoc 2y ago> Do you really think that a $400 cost is of any issue for military use? If your name is Ukraine then yeah. Effectively halves the number of drones you can build
- littlestymaar 2y ago> Effectively halves the number of drones you can build You're confusing the price tag of an FPV drone (for which this tech has no use, 4km precision is roughly the range of such drone, so even without a positioning device you'd get such a precision…) with the one of a long-range drone which is hundreds of magnitudes larger, even for Ukrainians.
- fullspectrumdev 2y agoFPV used in combat these days go a lot further than 4km, you can punch out to 25km+ with a good repeater setup easily, 10-15km with a good mast setup, and strikes using FPV quads out to 45km have been documented (but these are rare). You just need to plan your battery selection and consider the electronic warfare environment to go the distance. There’s also the optical fiber drones which come in spool lengths up to 20km…
- littlestymaar 2y ago
- rkagerer 2y ago4 km seems kind of coarse. Could you combine it with knowledge of satellite imagery or something to increase precision?
- anovikov 2y agoAn idea: use satellites for navigation. No, not the satellite signals, but the satellites themselves. Use NORAD orbital elements data for satellites to deduce land coordinates using time and pixel coordinates of satellites observed. Low orbit satellites will be only observable for two hours or so after sunset and before sunrise, but there are enough medium Earth orbit satellites that are still bright enough for a small camera and are visible whole night.
- nuccy 2y agoIf you see satellites then likely you see even more stars. Unlike satellites the stars barelly move (actually they do, see "proper motion" [1]) relatively to each other, so a catalogue of stars (two coordinates values and two proper motion values) along with the time of observation is sufficient to be used over decades, unlike NORAD orbit elements requiring regular updates. With stars you need just one image at a known time to find your location, with satellites it is much much more complicated: you need to know where the sun is, you need few images of a satellite or even a video (likely on top of image of stars anyway) to distinguish it from the stars and to solve the trajectory. 1. https://en.m.wikipedia.org/wiki/Proper_motion https://en.m.wikipedia.org/wiki/Proper_motion
- anovikov 2y agoHow do you find your location from one image of stars? It is possible if you have a precise vertical but you don't have a precise vertical on a moving UAV. That is, you need an inertial system on top that will provide you with a vertical. With satellite images, you don't need anything apart from time. And no, you don't need to "make a video to see satellites move", you start with your approximate location, make an image and find satellites within a circle where each of them might be, starting with the slowest moving - furthest away from you - ones (they provide poorest precision of coordinates because parallax is small, but you need to start with something, but their search circle will also be smaller), locating those, you get better coordinates of yours and the search circle for each satellite becomes smaller, then you can find faster moving satellites too to get precise coordinates of yourself.
- Animats 2y agoHere's the camera used.[1] It's not exotic. It's a 1936 × 1216 Sony sensor with a C-mount lens. That's below current phone camera resolution. It's monochrome, which makes sense in this application. They have bigger collecting optics than a phone, and you get better sensitivity without the color filters. I'm not clear on how they get their "down" reference. It's clear how they get heading; that's easy if you can see the stars. But you need an accurate horizon or vertical to get latitude and longitude. One degree of error in the vertical is maybe 100 km of error in position. How good are drone AHRS systems today in attitude? They have a correction system that works if you fly in a circle, but that just corrects for constant misalignment between camera and down reference. [1] https://www.alliedvision.com/fileadmin/pdf/en/Alvium_1800_U-240c_Closed-Housing_C-Mount_Standard_DataSheet_en.pdf https://www.alliedvision.com/fileadmin/pdf/en/Alvium_1800_U-...
- kragen 2y agoOne degree of error in the vertical for the drone's control system, if it's hovering by blowing air downward at 5 meters per second, would be a ground speed of 87 mm/s (sin(1°)×5m/s) in whichever direction the tilt is. Also without any correction in the propeller speed it would result in a loss of altitude averaging 0.76mm/s (2.7 m/hour, (1 - cos(1°) 5m/s). But that could also be caused by something like a mild downdraft, while the horizontal drift could be caused by an imperceptibly weak breeze. So I don't really know how this is normally done. If you can set the drone on the ground for a few minutes, you should be able to get a very good reference up-vector, but I don't know how long the MEMS gyros can preserve that up-vector without GNSS once it takes off. At sea you can probably look at the horizon with a camera unless it's foggy.
- leoc 2y agoA 2021 PopMech article about the US military's revival of interest in celestial navigation: https://www.popularmechanics.com/military/research/a36078957/celestial-navigation/ https://www.popularmechanics.com/military/research/a36078957... It mentions a handheld system designed for special forces units to use, but I assume that that would incorporate something like a camera gyro stabiliser, presumably making the calculations easier than when relying on "strapdown" sensors.